Can data be written to a disc if it already has data on it?

Yes, PowerBackup can continue a multi-session disc that already has data on it. By default, PowerBackup prompts a window to ask you whether you want to continue a written disc or erase the disc when you insert a ReWritable disc that has data on it. You can also simply go to the "Disc" page in "Backup Options" and set PowerBackup to continue a disc or erase a disc each time a written ReWritable disc is inserted.
Please note that PowerBackup always continue a disc during scheduled backup.